The basic hydrogen PEM fuel cell consists of two catalyst-loaded electrodes separated by a proton exchange membrane (PEM). Molecular oxygen supplied to the catalytically active cathode is dissociated and reduced to O2− (an energetically favored process). Molecular hydrogen supplied to the anode is dissociated and the hydrogen atoms oxidized to protons (H+), giving Up their electrons to the anode. Those electrons propagate through the external circuit to the cathode, delivering work in the process. The protons generated at the anode meanwhile diffuse through the PEM to combine with the reduced oxygen, producing water and heat as the waste products. Both the anode and cathode (in addition to the requirement that they be electrically conductive) are engineered with specific catalysts, commonly Pt, to facilitate the molecular dissociations and the respective electron transfers.
In a microbial fuel cell the anode need not be catalytically active, rather microbes in electrical communication with the anode are fed more complex fuels (e.g. carbohydrates such as glucose) that the microbes disassemble into protons in a series of reactions as part of their normal metabolic processes. Each proton extracted leaves behind an electron that is delivered to the anode and becomes available to do work as it traverses the external circuit. The protons meanwhile diffuse through the membrane to combine with reduced oxygen (generated at the cathode) as in the hydrogen PEM fuel cell. Microbial fuel cells have several advantages over conventional PEM fuel cells. Microbial fuel cells do not require pure molecular hydrogen as fuel and can use carbohydrate molecules which provide greater volumetric energy density as compared to molecular hydrogen. In addition, the anode need not be catalytically active which reduces cost since catalysts such as Pt commonly used as anode catalysts are expensive.
In hydrogen PEM fuel cells, the anode catalyst is in intimate electrical contact with the electrically conductive anode material being typically some form of conductive carbon resulting in low electrical impedance for electron transfer from the catalyst to the anode. For most microbial fuel cells, in contrast, the electrons are produced by metabolic processes inside the cell and must be transported across the high impedance cell membrane to the anode. Generally this is accomplished by molecules referred to as electron shuttling mediators. These are molecules that readily traverse the cell membrane, picking up electrons liberated within the cell and transporting them outside of the cell permitting passage to the anode through diffusion where their electronic charge is transferred.
Some microbial cells, in direct contact with the anode surface are able to transfer the electrons they generate directly to the anode. In either case a high surface area anode infiltrated with the microbial cells is beneficial. In the case of the direct electron transfer cells, because this maximizes the contact area with the electrode while in the case of electron shuttling mediators because this minimizes the distance over which the mediators must diffuse to transfer their charge to the anode.
Although microbial fuel cells have shown some promise, the slow transfer rate of electrons to the electrode has significantly limited power output of such fuel cells, and thus their commercial applicability. What is needed is a solution to significantly increase the transfer rate of electrons to the electrode to allow microbial fuel cells generate more power.